The invention relates to a unit comprising a high-pressure discharge lamp and an ignition antenna, which high-pressure discharge lamp is provided with a light-transmitting, gastight lamp vessel containing an ionizable filling, in which lamp vessel a first and a second electrode are arranged, and each one of the electrodes is connected to a current conductor of its own, which issues from the lamp vessel to the exterior, which ignition antenna, which is arranged near the lamp vessel, is provided with a gastight antenna vessel which contains an ionizable filling and which is provided with a further outer electrode, which is provided on an outside surface of the antenna vessel and connected to a further current supply conductor.
Such a unit comprising a high-pressure discharge lamp and an ignition antenna is disclosed in 99/48133. The known lamp has a comparatively short discharge arc, enabling light generated by the lamp to be satisfactorily focused. As a result thereof, the lamp can very suitably be used, inter alia, as a projection lamp, for example in a projection system or in a car headlamp system. The known lamp contains a filling, the pressure of which, during operation of the lamp, assumes a very high value of the order of several tens of bars and higher. To improve the ignition behavior of the known lamp, the lamp is provided with an ignition antenna in the form of a vessel filled with an ionizable gas, which vessel is provided with a capacitively coupled electrode. When an ignition voltage is applied to the further current conductor, the further outer electrode causes ionization of the ionizable filling of the antenna vessel. As a result, the filling of the antenna vessel has become conducting, thereby generating an electric field in the lamp vessel. In general, the ignition time is shorter as the ignition voltage applied to the ignition antenna is higher. This applies if the lamp is ignited in the cold state as well as if the lamp is re-ignited shortly after having been turned off, i.e. when the lamp is still hot. It has been found that in spite of the presence of the ignition antenna, a substantial degree of ignition delay may occur, which is a drawback.
Therefore, it is an object of the invention to provide a measure which, in the case of a unit as described in the opening paragraph, counteracts said drawback. To achieve this, the unit of the type described in the opening paragraph is characterized in accordance with the invention in that the antenna vessel of the ignition antenna also encloses an electroconductive element. Surprisingly, it has been found that ignition delay has substantially disappeared. When an ignition voltage is applied to the further current conductor, the further outer electrode substantially instantaneously brings about ionization of the ionizable filling of the antenna vessel, so that, also substantially instantaneously, like in the case a metal conductor is used for the antenna, an electric field is generated in the lamp vessel. As a result, ignition delay is counteracted, so that the ignition time is reduced.
The measure in accordance with the invention is particularly effective when high-pressure discharge lamps are ignited under unfavorable conditions, for example if the lamp has been in a dark environment for a substantial period of time.
In order to counteract optical losses of light emitted by the lamp, the antenna vessel is preferably made of a translucent material, for example a ceramic material such as monocrystalline metal oxide, for example sapphire, polycrystalline metal oxide, for example translucent, gastight aluminum oxide (DGA), yttrium aluminum garnet (YAG) or yttrium oxide (YOX), or polycrystalline non-oxidic material, such as aluminum nitride (AIN). Glass, for example quartz glass, can also suitably be used as the translucent material and has the additional advantage that it enables a comparatively great design freedom of the ignition antenna.
In the unit in accordance with the invention, the type and intensity of the radiation generated in the antenna vessel for achieving a short hot-state reignition time is not essential. To achieve a short ignition time when the lamp is ignited in the cold state, in the absence of ambient light, it is favorable, however, if the ignition antenna generates, in an activated state, UV radiation, preferably, in a wavelength range from 190 to 260 nm. For this purpose, for example, the ignition antenna contains a filling of mercury and argon.
As the further outer electrode is provided on the outside surface of the antenna vessel, a gastight lead-through to the electroconductive element enclosed in the antenna vessel is not necessary. In addition, this leads to an increased choice regarding the materials that can be used for the further electrode, because, in this case, the wall of the antenna vessel precludes any chemical interactions between the further electrode, the conductive element and the filling.
The ignition voltage applied to the ignition antenna is, for example, a high-frequency AC voltage, but it may alternatively be a pulse-shaped voltage, which may or may not be repetitive.
In a favorable embodiment, the unit in accordance with the invention is further characterized in that the electroconductive element is situated inside the antenna vessel at the location of the further outer electrode. If the antenna vessel comprises an elongated part along which the further electrode extends, it is attractive for the electroconductive element to extend at least 2 mm beyond the further electrode. In this case, an instantaneous ionization in the antenna vessel is guaranteed upon applying an ignition voltage to the further electrode.
An attractive embodiment of the unit in accordance with the invention is characterized in that the lamp vessel comprises a comparatively wide central portion and, on either side thereof, neck-shaped end portions, the electrodes being arranged in the central portion of the lamp vessel, and the current conductors each extending through a respective end portion, and the antenna vessel of the ignition antenna being a tube which is bent, near the central portion, around one of the end portions. This two-sided, high-pressure discharge lamp can be readily mass-produced on an industrial scale.
Preferably, the lamp forms part of a projection system, and the unit is provided with a reflector. A practical, compact embodiment of such a unit is characterized in that the reflector is a converging reflector with an optical axis, a light emission opening and, opposite said opening, a further opening, the reflector surrounding the central portion of the lamp vessel, the neck-shaped portions of the lamp vessel extending along the optical axis, and the end portion, around which the ignition antenna is bent, issuing to the exterior through said further opening.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.